Conventionally, there have been publications describing a process for producing a 4-alkylthiozolidine-4-carboxamide represented by the later-shown general formula (2) (for example, refer to Justus Liebigs Ann. Chem. (1966), 697, 140–157). However, there is no description of a process for obtaining a 2-alkylcysteinamide represented by the later-shown general formula (1) from the 4-alkylthiozolidine-4-carboxamide. In addition, the above publication describes a technique for obtaining a cysteinamide derivative, penicillaminamide, by treating a 5,5-dimethylthiazolidine carboxamide in concentrated hydrochloric acid, however, when this technique is applied to the 4-alkylthiozolidine-4-carboxamide represented by the later-shown general formula (2), a hydrolysis is excessively proceeded, and a large amount of 2-alkylcysteine is generated as a by-product. Because of the generated 2-alkylcysteine that has properties similar to those of the 2-alkylcysteinamide of interest represented by the later-shown general formula (1), it makes separation and purification difficult. Accordingly, the above technique is not preferable.
Moreover, a conventional process for producing an optically active 2-alkyl-L-cysteine comprises, for example, providing an optically active L-cysteine methyl ester as a starting material; cyclizing it with pivalaldehyde; protecting the cyclized product with formaldehyde; methylating the protected product with a lithium reagent and methyl iodide; opening the ring with hydrochloric acid; and deprotecting it, to obtain a 2-methyl-L-cysteine in the form of a hydrochloric acid salt (see U.S. Pat. No. 6,403,830, for example). Nevertheless, this process requires an optically active substance as a starting material and a large number of steps, and thus is complicated. This process also requires expensive reagents. Thus, it is hardly said that this is an industrially excellent process.
There have been no reports regarding a process for producing the optically active 2-alkyl-L-cysteine represented by the later-shown general formula (2) through a stereoselective hydrolysis of a 2-alkylcysteinamide represented by the later-shown formula (1) using an enzyme derived from a microorganism.